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Hornsby Shire Council |
Attachment to Report
No. WK34/09 Page 0 |
SHEPHERDS DRIVE,
CHERRYBROOK
ROUNDABOUT AT
KENBURN AVENUE
SUMMARY
The
redevelopment of Cherrybrook Village Shopping Centre in 2005 resulted in
increased traffic delays in Shepherds Drive. Traffic attempting to enter
Cherrybrook Shopping Village is delayed by vehicles slowing to access parking
spaces within the centre. To manage the problem a left turn slip lane into the
shopping centre was provided in August 2008. A review by Traffic and Road
Safety Branch completed in March 2009 has confirmed that delays have been
significantly reduced and the roundabout is operating in accordance with its
design parameters. Additional improvements cannot be justified using only
traffic management criteria.
Despite the
recent improvements to the roundabout some motorists consider the design too
restrictive. Council continues to receive correspondence concerning the
operation of the roundabout. Further
changes to widen the roadway will create an environment where vehicles can
negotiate the roundabout at speeds exceeding the current speed limit in
Shepherds Drive, nullifying the principal road safety benefit of the
roundabout.
PURPOSE/OBJECTIVE
This report has been prepared to review the design and operation of the
roundabout following modifications to reduce traffic delays as per the
resolution of reports WK49/07 and WK22/08, and to review options regarding the
likely cost and benefit of any further work.
DISCUSSION
Current roundabout arrangement.
A number of site inspections in 2006 and 2007 confirmed
that the single lane roundabout was prone to congestion caused by vehicles
attempting to enter the carpark queuing back into Shepherds Drive in both
directions, causing delays on all legs of the intersection. However, while
queuing did occur during traffic peaks, the extent and duration of the queuing
fell below the level required to justify major reconstruction. Traffic
management practice and Roads and Traffic Authority (RTA) modelling standards
require analysis of a continuous hour of traffic data to quantify and compare
delays across the Sydney area. Therefore locations experiencing delays of less
than one hour continuous duration are not suitable for cost benefit analysis
and will not qualify for state or federal funding assistance. Typically queues
in Shepherds Drive would form then disperse a number of times within a one hour
period, with the worst periods around mid day and 5 pm weekdays and Saturday
mornings.
Consultation between shopping centre management
and Council resulted in a number of changes to the shopping centre car park
commencing late 2005 which reduced but did not eliminate queuing into the
roundabout. In addition shopping centre management agreed to contribute 50%
towards the design and construction of “low cost” improvements to the
roundabout which it had no obligation to do as Council has accepted the traffic
assessment provided with their development application for the refurbishment
completed in 2005. At that time the developer’s traffic consultant provided a
report which showed that anticipated delays post redevelopment would be within
the acceptable range for traffic efficiency for the Sydney region, therefore
Council was unable to impose additional consent conditions on the developer in
relation to access. The veracity of the consultant’s data has since been
confirmed by Council after the redeveloped site commenced operating. Council’s
traffic data confirmed that traffic management standards were being met despite
continuing complaints from the public regarding the roundabout. Nevertheless, Council
considered report WK49/07 and resolved to investigate options.
In 2008 Council prepared a proposal to provide
an extra left turn slip lane to “desirable” traffic engineering standards which
required adjustment of a Telstra pit and property acquisition. The cost of this
work was estimated to be $270,000 excluding property acquisition. This proposal
was “Option 2” as reported to Council in WK22/08. As an alternative, a proposal
avoiding adjustment of the pit or property acquisition was prepared using
“minimum” traffic engineering standards. The cost of this proposal, “Option 1”
in WK22/08, was $45,000 and this option was adopted by Council and referred to
the Local Traffic Committee (LTC) for a recommendation.
It should be noted that the LTC operates under
RTA guidelines and has no decision making powers. Council decides which
proposals it will refer to the LTC, based on Council’s own funding priorities,
while the LTC’s role is to recommend whether these proposals meet acceptable
design, operation and enforcement standards. The LTC report did advise that an
alternative proposal meeting “desirable” standards had been considered by
Council but was not adopted due to excessive cost.
Accordingly the LTC recommended that the low cost left turn lane proposal
using minimum design standards was acceptable and should be provided. In
determining whether the use of minimum standards is appropriate in this
situation, it must be recognised that the additional left turn lane was
provided to store vehicles waiting to enter the shopping centre carpark without
obstructing through traffic during traffic peaks. While the lane widths are to
minimal standard they are sufficient to allow through vehicles to slowly pass
stopped vehicles. The approved design was developed using Australian Standard
vehicle templates and allows two cars to enter the roundabout at slow speed.
Where the left turning vehicle does not keep left, or either vehicle is larger
than the design vehicle, the other vehicle has the option of using the mountable
portion of the roundabout which is designed specifically to allow vehicles to
cross at slow speed. This creates a low speed environment which is appropriate
for the level of traffic congestion and pedestrian conflict occurring during
peak periods. Whether or not the queue forms is dependant on motorist’s
behaviour within the carpark and no amount of work on Shepherds Drive or the
roundabout can influence the need to queue within the carpark.
Traffic data obtained between
Attachment 4 shows the relative queue lengths before and after the
additional lane was added. A queue is defined as occurring when vehicles travel
at 10 km/h or less and for the most part these queues consist of moving
vehicles. It should be noted that even the worst queues measured in 2007 and
reported in LTC12/2008 equated to a Level of Service C. Level of Service is a
traffic management performance standard which uses quantitative data to provide
a qualitative assessment of roads and intersections. There are 6 Levels of
Service ranging from Level of Service A (minimum delay) to Level of Service F
(worst delay with traffic arriving faster than it can leave). Level of Service
C is considered satisfactory for new work in the Sydney Region therefore the
current arrangement, at Level of Service B, is better than merely
acceptable.
Alternative Options
Council’s Design and Construction Branch have investigated 2 options
relating to the operation of the roundabout however it needs to be recognized
that the queues in Shepherds Drive forming to enter the shopping centre carpark
are initiated within the carpark and no changes to the roundabout in Shepherds
Drive will reduce queues.
Option 1 - This option is
the alternative considered but not adopted in 2008 due to the higher cost. It
provides wider traffic lanes by relocating a Telstra pit and acquiring land for
an estimated cost exceeding $300,000. Queuing and delays would not change. The
benefit would be a reduction in complaints from the public wanting faster
movement through the roundabout. The disbenefit would be that vehicles, in
particular through vehicles, could enter the roundabout faster than currently,
leading to more complaints of vehicles not giving way, more complaints from
pedestrians about difficulty in crossing Shepherds Drive, more incidents of
high range speeding and possibly more crashes and crashes of greater severity.
Refer to Attachment 2 for a copy of this option.
Option 2 – A less
costly option to improve manoeuvring through the roundabout involves narrowing
the central roundabout island by about 1 metre to create a wider through lane
within the roundabout, without changing the kerb alignment outside the child
care centre. The estimated cost would be $50,000. Queuing and delays would not
change and the benefits and disbenefits are the same as above. Refer to
Attachment 3 for a copy of this option.
It should be noted that both options severely compromise the speed
reduction benefits of the roundabout outside peak times. The speed limit in
The Traffic Officer Eastwood Police advises that the Police will not
support any changes to the roundabout which will result in increased speeds.
Other sections of Shepherds Drive have a history of after hours speeding
complaints and the roundabout in its current form has proven to be an effective
speed management device with only one collision reported in the last 12 months.
Cherrybrook Village Shopping Centre management has advised it is unable
to consider financially contributing to further reconstruction at this stage as
it considers the left turn slip lane has reduced delays and complaints
significantly. Centre management will continue to consider changes to the
internal layout of the carpark to improve circulation and further reduce
queuing into Shepherds Drive.
The following options have not been considered in any detail and will
require extensive design and consultation if Council requires accurate
estimates.
Option 3 – Construct a
new roundabout with 2 circulating lanes. The geometry of the intersection is currently
appropriate for a single lane roundabout only. To provide a second lane will
require almost total reconstruction due to pavement cross falls, necessitating extensive
property acquisition from all corner properties (and perhaps further), and
additional public utility relocation. Pedestrian access will be severely
affected by increased speeds. Based on the estimate for Option 1, the cost of
this work will exceed $500,000.
Option 4 - Council may
consider signalising the intersection, either by signalising the roundabout or
removing the roundabout and reconstructing the intersection. The cost of either
would exceed $200,000. Neither option can be recommended as queues entering the
carpark will continue to form back into the intersection, preventing vehicles
from entering the intersection. In addition, traffic signals will result in
vehicle delays occurring outside peak periods as well as during peak periods.
Signals may, however, improve pedestrian access during peak periods. Assuming
Council could obtain RTA approval for the traffic signals, Council would have
to fully fund traffic signals as they do not meet the RTA warrants for traffic
volumes, pedestrian volumes or crashes. As stated earlier, the volumes and
delays have to be measured over an hour, and the required volumes have to be
exceeded for four hours in total each day.
Conclusion
The provision of the slip lane in mid 2008 resulted in the roundabout
operating in accordance with the design parameters which included reducing
delays during peak periods without increasing after hours speeding, at a
construction cost acceptable to Council. Delays currently experienced on
Shepherds Drive are being monitored and are within the range traffic management
guidelines consider acceptable throughout Sydney Region during peak periods.
Further changes to the roundabout cannot be recommended given that the current
layout meets the design goals and further changes may have adverse road safety
implications, particularly for pedestrians. Despite the level of complaints received
recently regarding the operation of the roundabout, on site observations and
available traffic data indicates that the intersection of Shepherds Drive and
Kenburn Avenue operates in accordance with prescribed traffic management
performance standards.